The cotter threads are the 26 TPI as every other one on this vintage of Superbe, 1974. Raleigh didn't use the Whitworth thread form, at least this far down the road. The thread diameter is different on these I checked for 1/4", too large; then 5/16" and smaller than that. I'm thinking maybe 9/32 which is on the BSC/CEI thread chart. The Churchill also seems to use the same cotters with fancy plated nuts. Normally I don't care much with consumable parts, but these bikes are in very good original condition and if it isn't too much bother I'd like to keep it that way and still sound enough for a trouble free ride. The OEM ones were soundly trashed by previous owners or shops.
I know where to get good cotters at Bikesmith Design, they make ones that fit like original with strong 7mm threaded stems; way better than the cheaper pressed ones that dominate the aftermarket.